Cells in a liquid culture must be concentrated or harvested before they can be preserved for later use, stained for direct analysis, or processed to isolate target specific materials therefrom. Most cell harvesting and concentration techniques involve centrifugation, filtration, or a combination of centrifugation and filtration. (See, e.g. Molecular Cloning, (1989) ed. by Sambrook et al., pp 2.22 and filtration system reference). Unfortunately, neither filtration nor centrifugation is amenable to automation. Specifically, neither can be performed at basic pipettor-diluter robotics stations, such as the Biomec.RTM.. When it becomes necessary to isolate or analyze certain types of material in the interior of a cell, such as a target nucleic acid or a protein, the cell membrane must be disrupted and the contents of the cell released into the solution surrounding the cell. Such disruption can be accomplished by mechanical means (e.g., by sonication or by blending in a mixer), by enzymatic digestion (e.g. by digestion with proteases), or by chemical means (e.g., by alkaline lysis followed by addition of a neutralization solution). Whatever means is used to disrupt a cell, the end product, referred to herein as a lysate solution, consists of the target material and many contaminants, including cell debris. The lysate solution must be cleared of as many of the large contaminants as possible before the target material can be further isolated therefrom. Either or both of the same two means described above, i.e. centrifugation and filtration, have been used to clear lysate solutions prior to further processing. However, for reasons given above, neither means of clearing a lysate solution is amenable to automation.
Many different systems of materials and methods have been developed for use in the isolation of nucleic acids from cleared lysate solutions. Many such systems are silica based, such as those which employ controlled pore glass, filters embedded with silica particles, silica gel particles, resins comprising silica in the form of diatomaceous earth, glass fibers or mixtures of the above. Each such silica-based solid phase separation system is configured to reversibly bind nucleic acid materials when placed in contact with a medium containing such materials in the presence of chaotropic agents. The silica-based solid phases are designed to remain bound to the nucleic acid material while the solid phase is exposed to an external force such as centrifugation or vacuum filtration to separate the matrix and nucleic acid material bound thereto from the remaining media components. The nucleic acid material is then eluted fiom the solid phase by exposing the solid phase to an elution solution, such as water or an elution buffer. Numerous commercial sources offer silica-based resins designed for use in centrifugation and/or filtration isolation systems, e.g. Wizard.RTM. DNA purification systems products from Promega Corporation (Madison, Wis., U.S.A.), or the QiaPrep.RTM. DNA isolation systems from Qiagen Corp. (Chatsworth, Calif., U.S.A.). Unfortunately, the type of silica-based solid phases described above all require one use centrifugation or filtration to perform the various isolation steps in each method, limiting the utility of such solid phases in automated systems.
Magnetically responsive solid phases, such as paramagnetic or superparamagnetic particles, offer an advantage not offered by any of the silica-based solid phases described above. Such particles could be separated from a solution by turning on and off a magnetic force field, or by moving a container on to and off of a magnetic separator. Such activities would be readily adaptable to automation.
Magnetically responsive particles have been developed for use in the isolation of nucleic acids. Such particles generally fall into either of two categories, those designed to reversibly bind nucleic acid materials directly, and those designed to reversibly bind nucleic acid materials through an intermediary. For an example of particles of the first type, see silica based porous particles designed to reversibly bind directly to DNA, such as MagneSil.TM. particles from Promega, or BioMag.RTM. magnetic particles from PerSeptive Biosystems. For examples of particles and systems of the second type designed to reversibly bind one particular type of nucleic acid (mRNA), see the PolyATract.RTM. Series 9600.TM. mRNA Isolation System from Promega Corporation (Madison, Wis., U.S.A.); or the streptavidin coated microsphere particles from Bangs Laboratories (Carmel, Ind., U.S.A.). Both of these systems employ magnetically responsive particles with streptavidin subunits covalently attached thereto, and biotin with an oligo(dT) moiety covalently attached thereto. The biotin-oligo(dT) molecules act as intermediaries, hybridizing to the poly(A) tail of mRNA molecules when placed into contact therewith, then binding to the streptavidin on the particles. The mRNA molecules are then released in water.
Indirect binding magnetic separation systems for nucleic acid isolation or separation require at least three components, i.e. magnetic particles, an intermediary, and a medium containing the nucleic acid material of interest. The intermediary/nucleic acid hybridization reaction and intermediary/particle binding reaction often require different solution and/or temperature reaction conditions from one another. Each additional component or solution used in the nucleic acid isolation procedure adds to the risk of contamination of the isolated end product by nucleases, metals, and other deleterious substances.
Various types of magnetically responsive silica based particles have been developed for use as solid phases in direct or indirect nucleic acid binding isolation methods. One such particle type is a magnetically responsive glass bead, preferably of a controlled pore size. See, e.g. Magnetic Porous Glass (MPG) particles from CPG, Inc. Lincoln Park, N.J., U.S.A.); or porous magnetic glass particles described in U.S. Pat. Nos. 4,395,271; 4,233,169; or 4,297,337. Nucleic acid material tends to bind very tightly to glass, however, so that it can be difficult to remove once bound thereto. Therefore, elution efficiencies from magnetic glass particles tend to be low compared to elution efficiencies from particles containing lower amounts of a nucleic acid binding material such as silica.
Another type of magnetically responsive particle designed for use as a solid phase in direct binding and isolation of nucleic acids, particularly DNA, is a particle comprised of agarose embedded with smaller ferromagnetic particles and coated with glass, e.g. U.S. Pat. No. 5,395,498. Yet another type of magnetically responsive particle designed for direct binding and isolation of nucleic acids is produced by incorporating magnetic materials into the matrix of polymeric silicon dioxide compounds, e.g. German Patent Application No. DE 43 07 262. The latter two types of magnetic particles, the agarose particle and the polymeric silicon dioxide matrix, tend to leach iron into a medium under the conditions required to bind nucleic acid materials directly to each such magnetic particle. It is also difficult to produce such particles with a sufficiently uniform and concentrated magnetic capacity to ensure rapid and efficient isolation of nucleic acid materials bound thereto.
Magnetically responsive beads designed for use in the isolation of target polymers, such as nucleic acids, and methods for their use therein are described in U.S. Pat. No. 5,681,946 and in International Publication No. WO 91/12079. These last beads are designed to become nonspecifically associated with the target polymer, only after the target polymer is precipitated out of a solution comprising the target polymer and the beads. Magnetic force is used to isolate the beads and polymer associated therewith from the solution. The magnetically responsive beads recommended for use in this last system are "finely divided magnetizable material encapsulated in organic polymer." ('946 Patent, col. 2, line 53).
A variety of solid phases have also been developed with ion exchange ligands capable of exchanging with nucleic acids. However, such systems are generally designed for use as a solid phase of a liquid chromatography system, for use in a filtration system, or for use with centrifugation to separate the solid phase from various solutions. Such systems range in complexity from a single species of ligand covalently attached to the surface of a filter, as in DEAE modified filters (e.g., CONCERT.RTM. isolation system, Life Technology Inc., Gaithersburg, Md., U.S.A.), to a column containing two different solid phases separated by a porous divider (e.g., U.S. Pat. No. 5,660,984), to a chromatography resin with pH dependent ionizable ligands covalently attached thereto (e.g., U.S. Pat. No. 5,652,348).
Materials and methods are needed which enable one to automate as many steps as possible to quickly and efficiently isolate target nucleic acids from cells or mammalian tissue. Specifically, methods and materials are needed for the concentration or harvesting of cells, for the clearing of solutions of disrupted cells or tissue, and for the isolation of target nucleic acids from such cleared solutions, wherein labor-intensive steps such as filtration or centrifugation are not required. The present invention addresses each of these needs. Nucleic acids isolated according to the present method can be used in a variety of applications, including restriction digestion and sequencing.